1. Technical Field
The present invention relates to a light-collecting device, a light-collecting device array, an exposure device and an image-forming apparatus.
2. Related Art
Conventionally, in a copy machine, a printer and the like for forming an image by an electrophotographic method, a laser raster output scanner (ROS) type exposure device for scanning light emitted from a laser light source by a polygon mirror has been used as the exposure device for writing a latent image on the photoreceptor drum. Recently, an LED-type exposure device, which uses a light-emitting diode (LED) as a light source is becoming mainstream in place of the laser ROS type exposure device. The LED-type exposure device is referred to as an LED print head and is abbreviated as LPH.
The LED print head is provided with an LED array and a lens array. The LED array is obtained by disposing a number of LEDs on a long substrate. The lens array is obtained by disposing a number of graded refractive index rod lenses so as to correspond to each LED. A number of LEDs are disposed in the LED array so as to correspond to the number of pixels in a main-scanning direction, such as 1200 pixels per 1 inch (that is, 1200 dpi). As the graded refractive index rod lens, a cylindrical rod lens represented by Selfoc (Registered trademark) is used.
In the LED print head, light emitted from each LED is collected by the corresponding rod lens to form an erecting equal-magnification image on the photoreceptor drum. Therefore, the laser ROS type scan optical system is not required, and this may be made significantly small as compared to the laser ROS type. Also, a drive motor for driving the polygon mirror also is not required, so that mechanical noise does not occur.
On the other hand, in the LED print head, a light path length (operating distance) from the LED to the image-forming point becomes shorter, so that an occupation rate of the exposure device around the photoreceptor drum becomes larger. That is, circumference of the photoreceptor drum is not congested with a longer operating distance, and an entire image-forming apparatus may be made smaller.
As a matter of course, by arranging the lens array between the LED array and the photoreceptor drum, the operating distance becomes longer than length of the cylindrical rod lens as compared to a case in which the LED array is arranged close to the photoreceptor drum. However, the operating distance of the rod lens is a few millimeters, and the operating distance of a few centimeters may not be obtained unlike in the case of the laser ROS type.
Meanwhile, as the electrophotographic exposure device, the LED print head using the LED array is common, so that the exposure method is commonly known as an “LED type”. However, it is not necessary to limit the light-emitting device to the LED, so that the “LED type” is appropriately referred to as a “light-emitting device array type” hereinafter.